254 
IOWA ACADEMY OF SCIENCES. 
to count the number of colonies. For this dilution one- 
tenth of a cubic centimeter of sewage is put into ten cubic 
centimeters of sterilized water, and one-tenth c.c. of this 
taken to make the culture. With the effluent no dilution 
has been made. Two methods of counting the plates have 
been employed. One is to divide the plates by means of a 
dividing circle into twenty equal divisions, counting three 
of these divisions, dividing by three to strike an average, 
and multiplying by twenty the number of divisions on the 
plate, and by ten, the denominator of the fractional part 
of a c.c. of sewage taken to make the culture. Of course, 
when dilutions were made the above result was multiplied 
by the denominator of the fractional part of a c.c. used, as 
to illustrate, 21+18+12 =51^3=17X20=840X10=8, 400X 
101=848,400. The above sample being diluted by ten c.c. 
of sterilized water to 1-10 of a c.c. of sewage. 
The other method is practically the same. The plate is 
divided into sixty square centimeters; three square centi- 
meters are averaged and multiplied by the number of 
square c.c. in the plate and the fraction of the denominator 
of the dilution. 
In each method care was taken to obtain a good average 
of the plate. As an illustration, if there was a spot where 
the colonies were especially thick or thin, counts were 
taken from them, and also from a spot containing about an 
average number of bacteria, if possible. 
The pipettes, petri dishes, etc., used in the work, were 
sterilized by dry heat for one hour and kept away from 
dust and moisture. 
The media used in these experiments has been, in the 
main, ordinary agar agar, gelatine having been used on sev- 
eral occasions to determine the variations between the 
number of colonies produced by agar and gelatine cultures 
respectively. It was found that on gelatine plates there is 
usually a slight increase in the number of colonies, but on 
account of the liquefying properties, it has not given as 
much satisfaction as agar cultures. 
Another method employed for the determination of gas 
producers is of special interest, as it can be shown by 
